Whenever we are involved in a creative activity that is self-rewarding, a feeling called “flow” overcomes us. When we are flowing we lose all sense of time and 　1　of what is happening around us; 　2　, we feel that everything is going just right.
　　A rock dancer 　3　his feeling of flow like this “If I have enough space, I feel I can 　4　an energy into the atmosphere. I can dance for walls. I can dance for floors. I become one 　5　the atmosphere.” “You are in an ecstatic 　6　to such a point that you don't exist,” says a composer. The same holds 　7　for players of sport, surgeons, and cave explorers, who enjoy their activity 　8　they can expect little material reward.
　　Flow provides a sort of physical 　9　along with an altered state of 　10　. “Your body feels good and your energy is flowing,” as one man 　11　it. In this state people are 　12　involved in what they are doing that they do not think of themselves 　13　separate from their activity. Moreover, they concentrate 　14　on their activity. Their sense of time is altered and they 　15　meals and sleep without noticing their loss. Sizes and spaces also seem 　16　.
　　It seems then that flow is a “floating action” 　17　the individual is aware of his actions but not aware of his awareness. 　18　people think about it, flow is destroyed, so they never ask 　19　questions such as “Am I doing well?” or “Did everyone see my jump?” So it is believed that to flow successfully depends a great deal on the activity itself; not too difficult to produce anxiety, not too easy to 　20　boredom; challenging, interesting, fun.
______

A.welfare

B.being

C.sustenance

D.maintenance